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Marine extinction intensity through time. The blue graph shows the apparent percentage (not the absolute number) of marine animalgenera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the "Big Five" extinction events are clickable hyperlinks; see Extinction event for more details. (source and image info)

The Ordovician–Silurian extinction event or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life[1]. Between about 450 Ma to 440 Ma, two bursts of extinction, separated by one million years, appear to have happened .[2] This was the second biggest extinction of marine life, ranking only below the Permian extinction. At the time, all known life was confined to the seas and oceans.[3] More than one hundred families of marine invertebrates died, including two-thirds of all brachiopod and bryozoan families[3]. Particularly affected were brachiopods, bivalves, echinoderms, bryozoans, and corals. The immediate cause of extinction appears to have been the continental drift of a significant landmass into the south polar region, causing a global temperature drop, glaciation, and consequent lowering of the sea level, which destroyed species' habitats around the continental shelves.[2] Evidence for this was found through deposits in the Sahara Desert. When Gondwana passed over the south pole in the Ordovician, global climatic cooling occurred to such a degree that there was widespread continental glaciation. This glaciation event also caused a lowering of sea level worldwide as large amounts of water became tied up in ice sheets. A combination of this lowering of sea level, reducing ecospace on continental shelves, in conjunction with the cooling caused by the glaciation itself are likely driving agents for the Ordovician mass extinction.[4]

Contents

History

The extinction occurred 443.7 million years ago, and marks the boundary between the Ordovician and following Silurian period. During this extinction event there were several marked changes in biologically responsive carbon and oxygenisotopes. This complexity may indicate several distinct closely spaced events, or particular phases within one event.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation.[6]

Possible causes

These extinctions are currently being intensively studied; the most commonly accepted theory is that they were triggered by the onset of a long ice age, perhaps the most severe glacial age of the Phanerozoic, in the Hirnantian faunal stage that ended the long, stable greenhouse conditions typical of the Ordovician. The event was preceded by a fall in atmospheric CO2, which selectively affected the shallow seas where most organisms lived. As the southern supercontinentGondwana drifted over the South Pole, ice caps formed on it. The strata have been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise; the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned, carrying diminished founder populations lacking many whole families of organisms. Then they withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992 p. 491). In the North African strata, Julien Moreau reported five pulses of glaciation from seismic sections. [7]

This incurred a shift in the location of bottom-water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom-water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

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Gamma ray burst hypothesis

Scientists from the University of Kansas and NASA have suggested that the initial extinctions could have been caused by a gamma ray burst originating from an hypernova within 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to high levels of ultraviolet radiation. This would have killed many species and caused a drop in temperatures.[8][9] While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred. One method would be to search the Moon for uneven exposure to gamma rays.

Adrian L. Melott et al estimated that gamma ray bursts from "dangerously close" supernova explosions occur two or more times every billion years, and this has been proposed as the cause of the end-Ordovician extinction.[10]

Volcanism and Weathering

A major role of CO2 is implied by recent research[11]. Through the Late Ordovician outgassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which sequestered CO2. In the Hirnantian Stage the volcanism ceased, and the continued weathering caused a significant and rapid draw down of CO2. This coincides with the rapid and short ice age.

End of the event

The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more. The rebound of life's diversity with the sustained re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

IGCP project

A major current (2004–08) project of UNESCO's International Geoscience Programme (IGCP), following a successful probe of the Ordovician biodiversification, has as its major objective to seek the possible physical and chemical causes, related to changes in climate, sea level, volcanism, plate movements, and extraterrestrial influences, of the Ordovician biodiversification, this end-Ordovician extinction, and the ensuing Silurian radiation [2].

See also

Sources

^ History Channel's Mega Disasters program, "Gamma Ray Burst", 2007, rebroadcast: 2008-11-13. The program attributes the "Ordovician extinction" (sic) explicitly as the second most grievously large extinction event after the Permian extinction.